The invention concerns a system for determining the airspeed of helicopters by using the cyclic and collective control position signals and the attitude angles for the pitch and roll attitudes.
The wide duty spectrum of modern helicopters demands increasingly accurate knowledge of the current flight condition, particularly the airspeed. Determining the airspeed in helicopters, however, is found to be difficult. Since the range of flight conditions includes extremely slow flight--extending to hover flight and even reverse flight--the principle of differential pressure measurement as a basis for velocity determination, well-proven in the case of aircraft, fails in this case. Furthermore, since helicopter flight can occur--even outside the low speed range--at very large total angles of incidence and yaw, rigidly installed velocity sensors, such as are used in aircraft, are inadequate. The installation of conventional incidence and yaw angle instruments is not permissable either because, in helicopters, it is almost impossible to find a measurement location suitable for a wide range of flight conditions; this is because all the components are affected by the main or tail rotor airflow to a greater or less extent.
The high level of interest in an airspeed system for helicopters--but also the difficulty of speed determination--is sufficiently indicated by the number and variety of the various measuring methods described below. Both the currently used sensor systems and the indirect systems being developed, the so-called analytic methods, satisfy the above accuracy requirements only at great expense or for limited flight ranges so that an improved system seems desirable.
For the reasons quoted above, systems have been developed specially for low speed flight in helicopters in order to determine the airspeed and, among these, it is possible to distinguish between relatively direct and purely indirect measuring methods.
The more direct methods include, for example LORAS--Low Range Airspeed System. The sensor consists of two venturi nozzles mounted at the ends of a rotatably supported arm and connected to a differential pressure signal generator, these rotate at 720 r.p.m. and are driven by a motor. A typical installation location is above the rotor head.
When flow arrives at the system rectangularily to the axis of rotation, the difference in the depression in the two venturi nozzles causes, during one revolution, a sine-shaped signal whose maximum amplitude is proportional to the translational velocity and whose phase position determines the yaw angle. The evaluation of the measurement with respect to magnitude and phase takes place in a computer unit.
The more direct methods may also be considered to include the LASSIE SYSTEM--Low AirSpeed Sensing Indication Equipment--a pilot and incidence probe with a gimbal suspension, a permissable incidence angle range of 0.degree..ltoreq..alpha..ltoreq.360.degree. and a permissable yaw angle range of -60.degree..ltoreq..beta..ltoreq.60.degree.. The probe is mounted on an outrigger at the side of the fuselage under the main rotor.
In LASSIE, the slow speed flight and high speed flight operating ranges have to be distinguished. In slow speed flight, the sensor is in the rotor downwash. The quite high downwash velocity of the rotor is superimposed on the airspeed at this point so that the dynamic pressures can be satisfactorily resolved. The longitudinal and transverse components u.sub.A and v.sub.A can be calculated from the arriving flow angles .alpha.,.beta. and the corresponding trigonometrical relationships. In high speed flight, the sensor is outside the rotor downwash so that the vertical component w.sub.A can also be determined.
Whereas, in high speed flight, the measuring principle of the LASSIE system can be directly compared with the corresponding sensors on winged aircraft, in low speed flight, conclusions on the airspeed are drawn from the arriving flow resulting from the airspeed and the rotor downwash. Investigations have shown that the LASSIE system provides different levels of accuracy depending on whether the sensor is inside or outside the rotor downwash. If lateral and vertical flight conditions are also taken into account, the transition range, in which the LASSIE sensor is in the region of the rotor vortex and supplies very erroneous results, is quite large. The complicated flow relationships at the measurement location make extensive calibration of the LASSIE system necessary. The calibration results make it possible to compensate for the errors as a function of the flight condition.
Both the LORAS and LASSIE systems are available in series versions and are used on various types of helicopters. Disadvantages of both systems, apart from the currently mediocre accuracy, are weight, complicated structure and high price. In addition, the systems are very vulnerable and redundancy can only be achieved at substantial expense.
As an alternative to the systems quoted, methods have appeared in recent years in which the airspeed is calculated from the measurement parameters of control positions, attitude angles and rotational velocities. Such systems are the VIMI method--Vitesse Indiquee par Moyen Internes, the FAULKNER method and LAASH--Litef Analytical Airspeed System for Helicopters. A fundamental advantage of these systems is the low vulnerability.
The VIMI method is based on the trim equations of the helicopters, which describe the equilibrium between the weight, drag and thrust forces. In order to determine the airspeed components, the trim equations are greatly simplified and rearranged; the longitudinal u.sub.A velocity component can then be described as a function of the longitudinal control .delta..sub.L and the pitch angle .theta. and the transverse velocity component v.sub.A can be described as a function of the lateral control .delta..sub.Q and the roll angle .PHI.: EQU u.sub.A =h.sub.1 .delta..sub.L +h.sub.2 .theta., (1) EQU v.sub.A =h.sub.3 .delta..sub.Q +h.sub.4 .PHI.. (2)
In the further developed Super-VIMI version, additional dependencies on the collective control .delta..sub.K, the helicopter mass m and the air density p are taken into account in both equations. Details of these additional terms are not known.
From company information, the accuracies achieved are very good. The extent to which the method retains its validity in the case of transient flight conditions and during vertical flight is not made clear in the documentation available but sacrifices in accuracy are probable. It may also be expected that that uneven input signals--due, for example, to the large amount of adjustment activity on the part of the pilot necessary to control the weakly stable helicopter behaviour--will make the results substantially worse. Filtering of the input control signals is proposed to improve the "selectivity" of the corresponding measurement parameters; filtering generally causes a substantial deterioration in the time behaviour.
Investigations have also shown that the separate calculation of the longitudinal and lateral velocity (VIMI) is not sufficient to achieve satisfactory longitudinal and lateral velocity accuracies over a large airspeed range because of the coupling between the longitudinal and lateral motion of the helicopter.
In the FAULKNER method, it is assumed that the reaction of the rotor system permits usable conclusions about the airspeed in the lower speed range. The method is based on the differential equation for the flapping motion of the rotor blades.
The quasi-steady system of differential equations for the flapping motion is solved and resolved into longitudinal and lateral velocity components, i.e it is inverted. The input parameters in this quite highly developed system are the cyclic and collective control positions, the rotational speeds and the rotor mast moment; these are used to calculate the flap coefficients.
This method requires a very good insight into the flight mechanics and aerodynamics relationships of the helicopter rotor. In order to obtain solutions, additional empirical assumptions with respect to the flow through the rotor are necessary. Measurement traces from flight tests using the "FAULKNER" method indicate satisfactory agreement between the speed curves. The lateral velocity steady state accuracy is not completely satisfactory. Here again, no information is available on the extent to which the method remains usable in climb and descent flight.
Reference may be made at this point to the fundamental problem of the high level of sensitivity to noise in the input signals when inverting the systems of equations. In the present case, noise should also be understood to mean high frequency position signal alterations. In the measurement traces, this is reflected in the estimated airspeeds, which are sometimes very noisy when compared with the reference velocities from the Doppler system.
The LAASH method only applies to slow horizontal flight because it is only under these conditions that, for a known flight weight, there is an unambiguous relationship between the collective control position and the magnitude of the airspeed. For a known roll angle, characteristic control curves against the yaw angle can be produced as a function of the magnitude of the flight speed.
In the LAASH method, the magnitude of the flight speed determined from the collective control signal is used to select the control characteristics (allocation of cyclic control to yaw angle) appropriate to this speed from a data file and hence to determine the yaw angle. Ambiguities exist in both the longitudinal and lateral control curves, i.e. various yaw angle values are possible. Because of the phase shift between the two control curves, it is possible to decide on an appropriate yaw angle by comparison.
The collective control positions in the hover and transition ranges (25 m/s.ltoreq.V.ltoreq.35 m/s) have only small gradients against airspeed, which is itself sufficient to make the determination of the magnitude of the flight speed subject to errors. Precisely in the hover range, furthermore, the allocation of the cyclic control positions to the yaw angle depends greatly on the flight speed so that, overall, the sensitivity to measurement errors and changes in parameters is very large.
Disadvantageous in this method is the ambiguity of the control positions with respect to the flight speed; thus, for example, the same collective control position is necessary in hover and at a airspeed of approximately 45 m/s. Since LAASH takes no account of this ambiguity, a preliminary estimate of the current speed range is necessary. LAASH is therefore associated with two severe conditions which greatly limit its practical usability.
In the indirect (analytical) methods, therefore, the airspeed is determined by using flight mechanics relationships. In all the methods, only the steady state or quasi-steady relationships are used. In the end, all the methods are based on incomplete flight mechanics models so that more or less substantial errors appear--depending on the maneuvers flown.
Summarising, the following can be concluded with respect to the systems and methods described:
All the sensor systems are directly affected by the flight condition of the rotor. The strong dependence of the rotor flow condition on the flight maneuvers has an unfavourable effect on the accuracy of the sensor system and, in some cases, requires error compensation which depends on the flight condition.
The analytical methods exploit the fact that in slow flight, large control changes lead to small changes to velocity and direction. An advantageous feature of the analytical methods is the low vulnerability, because all the sensors necessary are accomodated in the fuselage.
Analytical methods have advantages in principle over solutions depending on technical devices. The known analytical methods, however, can only be used with limitations--LAASH and the "FAULKNER" method only apply in hover and slow speed flight and even in the case of the VIMI method, it is to be expected that the non-linear curves of the control deviations and positional angles against airspeed may lead to substantial sacrifices in accuracy if not, in fact, to the failure of the method.
In none of the methods is the vertical velocity component (and the total angle of incidence) determined and the airspeed determination is, therefore, incomplete.